1. Field of the Invention
This invention relates to transmission and reception of electromagnetic signals in a medium. More particularly this invention relates to the multiplexing and demultiplexing of multiple signals from a cable. Even more particularly this invention relates to the separation simultaneously bi-directional light signals into its component incoming and outgoing signals.
2. Description of Related Art
Wavelength division multiplexing (WDM) is rapidly becoming the means for expanding the bandwidth or amount of information (data, telephony, or video) transported on fiber optic cables. Originally, the data modulated a single frequency or wavelength of light for transmission. The bandwidth being increased by employing time domain multiplexing (TDM) of the data signals. As the capacity of the installed base of the fiber optic cables became saturated with transmission of data, a method for increasing the capacity of the fiber optic cable became desirable.
The method developed to increase bandwidth use of fiber optic cable was wavelength division multiplexing. Light signals of different wavelengths are applied to a single fiber optic cable. The easiest, most cost effective method for wavelength division multiplexing is termed coarse wavelength division multiplexing (CWDM). Coarse wavelength division multiplexing employs light signal wavelengths that are widely separated to minimize light signal interaction or cross talk and allow the use of light signal separation techniques that are relatively simple. The light signal wavelengths are chosen such that the materials utilized have minimum dispersion and attenuation. The current preferred light signal wavelengths are 850 nm, 1310 nm, and 1550 nm.
Implementation of the wavelength domain multiplexing requires a multiplexer to combine the light signal wavelength for transmission of the combined light signals and a demultiplexer to separate the received light signals into the individual component light signals. Further, it is desirable to simultaneously transmit and receive signals from both ends of a fiber optic cable. This requires a combination of a multiplexer and demultiplexer to be present at both ends of the fiber optic cable. In fiber optic communication systems using two light signal wavelength such as 1310 nm and 1550 nm, the multiplexer/demultiplexer that separates the two bi-directional light signals is referred to as diplexers. Such companies as Koncent, Inc., Oplink Communications, Inc., and Harmonic, Inc. market diplexers or coarse wavelength division multiplexers that combine monochromatic light signals in the 1310 nm and 1550 nm wavelength range to form a bi-directional polychromatic light of the combined wavelengths and separate the bi-directional polychromatic light into the separate 1310 nm and 1550 nm wavelength light signals.
U.S. Pat. No. 5,673,342 (Nelson, et al.) teaches an optical fiber communication system having an optical fiber filter that can be manufactured at low cost and that can be conveniently incorporated into the system, substantially like a conventional fiber jumper. The filter comprises a length L of axially uniform optical fiber selected to have substantially no loss (e.g., <1 dB) at a wavelength λ1, and to have relatively high loss (e.g., >20 dB) at a wavelength λ2. The length L will typically be less than 100 m. In one embodiment the optical fiber is a single mode optical fiber at λ1 (e.g., 1.3 μm) that does not have a guided mode at λ2 (e.g., 1.55 μm). In another embodiment the fiber contains a dopant that does not substantially absorb radiation of wavelength λ1, but substantially absorbs at λ2. In the second embodiment, λ1 can be greater than λ2.
U.S. Pat. No. 6,289,148 (Lin, et al.) teaches a free-space micro-mirror wavelength add/drop multiplexer with full connectivity for two-fiber ring networks. The free-space nature of the switch mirrors allows use of the front and back sides of the mirrors for reflecting signals. According to one embodiment of the present invention a wavelength add/drop multiplexer is provided in which micro machined switch mirrors are arranged in a polygonal (e.g., hexagonal) geometry, which allows full connectivity. According to one embodiment a wavelength add/drop multiplexer is provided for deployment in a unidirectional two-fiber optical network including service and protection fiber routes. According to this embodiment the wavelength add/drop multiplexer includes a first input port for receiving a wavelength division multiplexed signal from the service fiber route and a second input port for receiving a wavelength division multiplexed signal from the protection fiber route. The wavelength add/drop multiplexer also includes a first output port for transmitting a wavelength division multiplexed signal to the service fiber route, a second output port for transmitting a wavelength division multiplexed signal to the protection fiber route, a third input port for receiving local signals from a local access port and a third output port for dropping signals to a local access port. The wavelength add/drop multiplexer further includes a reconfigurable switching matrix comprising a plurality of free-space micro mirrors, for performing routing of signals from the various input ports to the various output ports. According to an alternative embodiment a wavelength add/drop multiplexer is provided for deployment in a bi-directional two-fiber optical network including two service/protection routes.
U.S. Pat. No. 6,289,155 (Wade) discusses wavelength division multiplexing/demultiplexing devices using dual high index of refraction crystalline lenses. The wavelength division multiplexing device comprises a crystalline collimating lens for collimating a plurality of monochromatic optical beams, a diffraction grating for combining the plurality of collimated, monochromatic optical beams into a multiplexed, polychromatic optical beam, and a crystalline focusing lens for focusing the multiplexed, polychromatic optical beam.
U.S. Pat. No. 6,339,663 (Leng, et al.) provides a bi-directional wavelength division multiplexed optical communication system having bi-directional optical service channels. The bi-directional WDM optical communication system includes a bi-directional optical waveguide configured to carry a bi-directional optical communication signal comprising counter propagating WDM optical signals. Each WDM optical signal includes plural optical channels and an optical service channel. A bi-directional optical add-drop multiplexer optically communicates with the waveguide. A first optical service channel selector optically communicates with the first bi-directional optical add-drop multiplexer input/output port. The first optical service channel selector is configured to separate the first optical service channel from the first WDM optical communication signal such that the first WDM signal enters the first input/output port of the bi-directional optical add-drop multiplexer and the first optical service channel is routed to a service channel module. Similarly, a second optical service channel selector optically communicates with the second input/output port of the bi-directional optical add-drop multiplexer and routes the second optical service channel to a service channel module.
U.S. Pat. No. 4,776,660 (Mahlein, et al.) teaches a light branching element or diplexer comprising a first bi-directional light connection and a second and third unidirectional light connection. The unit is formed by a block having a straight surface groove with an embedded glass fiber which fiber is interrupted by a partially transmissive mirror lying on a slanting plane relative to the axis of the fiber. The light sensitive location of a light receiving semiconductor element is secured to the block adjacent to the mirror and the plane of the mirror is selected so that its normal extends out of the block at an angle of incidence smaller than 45° to the axis of the fiber to reduce reflections from the semiconductor member back to the mirror and into the fiber.
U.S. Pat. No. 5,144,637 and U.S. Pat. No. 5,031,188 (Koch, et al.) present a diplex lightwave transceiver that achieves full duplex light wave communications. The diplex transceiver is realized in a semiconductor photonic integrated circuit having an inline interconnecting waveguide integral with the transmitting and receiving portions of the transceiver. Semiconductor lasers and detectors operating at different wavelengths permit diplex or wavelength-division-multiplexed operation. In the transceiver, light wave signals from the laser propagate through the detector without interfering with the detector operation or the light wave signals being detected.
U.S. Pat. No. 5,712,864 (Goldstein, et al.) discusses a semiconductor photonic diplex transceiver. The photonic diplex transceiver includes a laser to generate a first optical signal having a certain wavelength and a photodetector to detect a second optical signal having another wavelength. The diplex transceiver also includes an absorber of the first signal disposed between the laser and the detector, which form integral parts of an optical waveguide. The laser generates the first signal in the form of a continuous wave and is disposed between the absorber and a selective modulator of the first signal. This reduces the problems of optical and electrical crosstalk between the transmit and receive functions.
“Bi-directional Single Fiber Links for Base Station Remote Antenna Feeding,” Steiner et al., European Conference on Networks & Optical Communications NOC 2000, Jun. 6–9, 2000, Stuttgart, Germany, discusses a bi-direction module employing a WDM beam splitter.
“1.3/1.55 Microns Duplex-Diplex Optical Transmission: The Brazilian Technology,” Celaschi, et al., Telecommunications Symposium, 1990. ITS '90 Symposium Record, SBT/IEEE International, pp. 454–457, September 1990, Rio de Janeiro, Brazil, presents results from the first experimental Brazilian route in which two separate optical channels have been combined in both duplex and diplex transmission. The experiments were demonstrated over 18 km of standard single-mode fiber using 1.29 and 1.52 micron edge-emitting laser diodes at 34 Mbit/s. The optical emitters and detectors were linked to the single mode fiber through specially designed wavelength division multiplexing couplers. The results indicate that either duplex or diplex transmission can be implemented in any installed standard single mode route up to 40 km.
“A 1.3/1.55 μm Wavelength-Division Multiplexing Optical Module Using a Planar Lightwave Circuit for Full Duplex Operation,” Hashimoto, et al.”, Journal of Lightwave Technology, IEEE, November 2000, Volume: 18 Issue: 11, pp. 1541–1547, discusses development of a hybrid integrated optical module for 1.3/1.55 μm wavelength-division multiplexing (WDM) with full-duplex operation. The optical circuit was designed to suppress the optical and electrical crosstalk using a wavelength division multiplexing filter, and an optical crosstalk of −43 dB and an electrical crosstalk of −105 dB were achieved with a separation between the transmitter laser diode and the receiver photodiode of more than 9 mm.
“Planar Lightwave Circuit Platform with Coplanar Waveguide For Opto-Electronic Hybrid Integration,” Mino, et al., Journal of Lightwave Technology, IEEE, December 1995, Volume: 13 Issue: 12, pp. 2320–2326 describes a planar lightwave circuit (PLC) platform constructed on a silica-on-terraced-silicon (STS) substrate for opto-electronic hybrid integration. This platform consists of an embedded silica PLC region, a terraced silicon region for optical device assembly, and a high-speed electrical circuit region. In the electrical circuit region, the coplanar waveguides (CPW) are prepared on a thick-silica/silicon substrate. This structure reduces the propagation loss of the CPW drastically to 2.7 dB/cm at 10 GHz, because the loss tangent (tan Δ) of the dielectric constants of silica is much smaller than that of silicon.